The present invention relates to a high frequency heating apparatus and more particularly, to a high frequency heating apparatus wherein improvements are made in the microwave feeding method and microwave radiator or antenna for varying the electric wave radiation pattern into the heating cavity with respect to time, thus advantageously achieving a more uniform heat distribution in space, facilitation of apparatus design through the possibility of separate analysis of the electric field distribution and the electric wave output, and an improved efficiency of electric wave output during small load operation.
When a conventional high frequency heatin apparatus, for example, a microwave oven, is investigated, there are various problems which have not been fully studied.
Firstly, one such problem is that the electric field distribution within the heating cavity is not sufficiently uniform. In other words, when electric waves are radiated into the heating cavity composed of electrically conductive material, standing waves are developed, with the cavity acting as a resonant cavity through interference between the incident waves radiated from the oscillator and the electric waves reflected through insufficient absorption thereof by the load. The modes of these standing waves are mainly determined by the dimensions of the heating cavity and the position at which the oscillator is installed. Meanwhile, the high frequency electric waves are radiated onto the load, with a radiation pattern corresponding to directivity of the electric wave radiator or antenna, the main components of which radiation pattern is determined by configuration and dimensions of the antenna. Since both the standing waves and radiation pattern are substantially fixed in space unless some particular countermeasures are taken, portions having a strong electric field and portions having a weak electric field are simultaneously present in the heating cavity, thus resulting in uneven heat distribution therein.
In order to cope with the above described problems, there have conventionally been proposed and put into practice a variety of countermeasures, the outstanding ones of which are installation of stirrers or employment of a rotatable table to place the object to be heard thereon. Neither of these, however, is a fundamental resolution of the problems.
Secondly, another problem involved in the conventional high frequency heating apparatus is that the degree of heating tends to differ between the upper portion and lower portion of the object to be heated such as food. For example, when milk or the like kept in a bottle is heated, it is often experienced that the milk in the upper portion of the bottle is too hot to drink, while the milk at the lower portion remains cold. This inconvenience is attributable to the radiation pattern described above, and mainly caused by the oscillating portion of the heating apparatus being strongly heated due to the installation of the oscillator at the upper part of the apparatus. To overcome the above described disadvantages, provision of the oscillating portion at the lower portion of the apparatus is considered. This arrangement, however, still has a disadvantage that since the distance between the oscillating portion and the object to be heated must be short for efficient utilization of the heating cavity. It is extremely difficult to make the electric field distribution on a planar or flat surface uniform, thus the concept is actually applied only to limited kinds of apparatus.
Thirdly, there is a further problem involved in the designing of such a known high frequency heating apparatus. This problem is that the analysis required to make the heating uniform cannot be separated from that to improve the electric wave efficiency through proper adjustment of the working point of the oscillator. When the problem as described above is considered, for example, with respect to a heating apparatus having stirrers or rotatable tables mentioned earlier, with these stirrers being designes mainly for uniform heat distribution, the configuration or mode or movement of the stirrer simultaneously causes variations with time of the substantial impedance, thus resulting in a large deviation from the optimum working point as observed from the oscillator. These drawbacks consequently bring about further problems such as insufficient output in spite of relatively favorable distribution or unsatisfactory distribution despite ample output, thus at the present state of the art making it necessary for the design to be a suitable compromise between these factors.
Fourthly, a still further problem of efficiency reduction is involved in the known high frequency heating apparatus. In the general practice, the rated electric wave output of a microwave oven is specified by the electric power consumed during a temperature rise with respect to a water load of 2000 c.c. It is commonly known in this line of industry, however, that when a water load of 100 c.c. is reached, the electric wave output is reduced to 50 to 60% of the rated output. Such a discrepancy may be avoided through an intended reduction of the rated output. These countermeasures, however, are not desirable from the view point of efficient utilization of the energy, and thus do not present fundamental resolution of the problems involved.
Most of the foregoing problems in the known high frequency heating apparatuses are mainly attributable to the power supplying systems for supplying electric waves into the heating cavity. Typical power supplying systems are briefly described hereinbelow. The known power supplying systems currently put into actual use may be broadly divided into a direct coupling system wherein the oscillator is directly coupled to the heating cavity or heating chamber, such as those disclosed in U.S. Pat. Nos. 2,763,757 and 2,813,185, a coaxial power supplying system, for example, those described in U.S. Pat. Nos. 2,632,090 and 3,221,132, and a wave guide power supplying method such as those detailed in U.S. Pat. Nos. 2,761,942 and 2,909,635. These prior art power supply systems, however, have merits and demerits as described hereinbelow.
Reference is made to FIGS. 1 to 3 showing schematic side sectional views of conventional microwave ovens (outer casings removed for clarity) employing the above described known power supply systems. In the heating apparatus of FIG. 1 employing the direct coupling system, an oscillator or magnetron m is directly mounted on the top wall ha of the oven walls defining the heating cavity or heating chamber h with an antenna a of the magnetron m extending into the heating cavity h for supplying high frequency energy thereinto. This arrangement, however, has a serious disadvantage in that matching or tuning of the oscillator and the load cannot be properly achieved when the dimensions of the heating cavity h as a resonant cavity and the position of the antenna a are defined, although advantageous in that a higher efficiency may be expected due to the absence of loss factor such as a waveguide (not shown) and that the antenna portion a having a rod-like shape is readily analyzed for its radiation pattern and exciting mechanism. This implies that it becomes extremely difficult in designing to simultaneously achieve uniform heat distribution in the heating cavity h and utilization of the oscillator m at a high efficiency.
In the waveguide power supplying system shown in FIG. 2, the electric waves radiated from the antenna a of the magnetron m which is mounted at one end of the waveguide w are propagated through the waveguide w disposed on the top wall ha of the heating chamber h and supplied into the chamber h through a rectangular opening wo formed in the other end of the waveguide w disposed on the top wall ha of the heating chamber h and supplied into the chamber h through a rectangular opening wo formed in the other end of the waveguide w. This opening wo has its width approximately equal to the width of the waveguide w. In the above arrangement, although the problems encounted may be smaller since the matching or tuning can be controlled outside of the heating cavity h, control of the radiation pattern is difficult because the radiation is effected through the opening wo of the waveguide w. More specifically, even if the opening wo is formed in the central portion of the wall ha of the heating cavity h, the radiation characteristics are still asymmetrical, thus making it extremely difficult to arrange the electric field to be evenly distributed within the heating cavity h.
Meanwhile, in the coaxial power supplying system of FIG. 3, the electric waves from the magnetron M are propagated through space between an outer conductor wa and an inner conductor wb into the heating cavity h for supplying high frequency energy thereinto. Although the above described coaxial power supplying system is advantageous as compared with the foregoing two systems of FIGS. 1 and 2 in the ease of matching and adjustment of the radiation pattern, the same system has disadvantages in that accurate dimensions of the outer conductor wa and inner conductor wb are particularly required due to the continuous construction of the inner conductor wb from the antenna of the oscillator m to the interior of the heating chamber h. The assembly of a number of components, i.e., the outer conductor wa, the inner conductor wb and the oscillator m in a predetermined relation gives rise to a further serious problem especially in the case of mass-production, thus adversely affecting the working efficiency.
Similarly, there have conventionally been proposed various arrangements for uniformly heating the object placed in the heating cavity, such as the stirrer method employing a vane or disk to be rotated in the heating cavity or the rotating table method for rotating, within the heating cavity, the object to be heated which is placed on the table. Each of these countermeasures, however, is of a secondary nature, and are not sufficient for the purpose of achieving the uniform heating of the object to be heated.
Meanwhile, an apparatus having a positive countermeasure of a primary nature for effecting the uniform heating has conventionally been proposed, for example, by U.S. Pat. No. 2,961,520. In that apparatus which has a coaxial power supplying system, as is clear from the statement in column 2, line 27 and after of the specification thereof, the junction between the fixed inner conductor extending from the magnetron and the inner conductor rotating at the heating cavity comes into question. More specifically, the arrangement of said U.S. Pat. No. 2,961,520 still presents various problems arising from limitations of the power supplying system, such as undesirable spark discharge at the junction, complicated choke construction, the necessity of precise dimension control and the countermeasures required upon adhesion of dirty matter to the junction.
Furthermore, in the conventional microwave ovens referred to in the foregoing description, it is a general disadvantage that, during heating through electric waves, the heat source is not visible with the eyes, thus psychologically giving rise to some uneasiness on the part of the user.